Scandium has the lowest atomic number among the rare earth elements and is used as a material for metal halide lamps, an additional element in alloys, an additional element in catalytic ceramics, and the like. However, scandium is expensive and has limited output, and further separation and purification are difficult. The use of scandium has been therefore limited.
It is incidentally known that nickel oxide ores, such as laterite ore, contain minute amounts of scandium. Scandium contained in nickel oxide ores can be recovered, for example, from a leachate obtained by the addition of sulfuric acid to a nickel oxide ore and then pressure leaching.
In Patent Document 1, for example, it is shown that nickel and scandium can be recovered from an oxide mineral by (a) the leaching step of obtaining a leachate containing nickel and scandium by the leaching of the oxide mineral with an acid under high temperature and high pressure, (b) a first neutralization step of removing iron and aluminum in the leachate as precipitates by adding a neutralizer to the leachate to adjust the pH to a range of 2 to 4, (c) a second neutralization step of, by adding a neutralizer to the solution after precipitates are removed in the first neutralization step to adjust the pH to a range of more than 4 to 7.5, recovering scandium in the solution as precipitates, and (d) a third neutralization step of recovering nickel in the solution as precipitates by further adding a neutralizer thereto to adjust the pH to more than 7.5.
In Patent Document 2, it is shown that high purity scandium oxide is obtained by adding an organic solvent to an aqueous phase scandium-containing solution containing at least one or more of iron, aluminum, calcium, yttrium, manganese, chromium and magnesium along with scandium to extract scandium components into the organic solvent, then, in order to separate the minor components extracted with scandium in the organic solvent, by carrying out scrubbing by adding an aqueous solution of hydrochloric acid thereto to remove the minor components, followed by adding an aqueous solution of sodium hydroxide to the organic solvent to change the scandium remaining in the organic solvent into a Sc(OH)3-containing slurry, dissolving Sc(OH)3 obtained by filtering this slurry in hydrochloric acid to obtain an aqueous solution of scandium chloride, adding oxalic acid to this to obtain scandium oxalate precipitates, filtering the precipitates to separate minute amounts of impurities to the filtrate, and then calcining the precipitates.
Patent Document 3 discloses an extraction agent, which has a diglycol amic acid backbone and is called DODGAA. This extraction agent is characterized by having very low solubility in water, being able to be completely disposed of by incineration, having the excellent rare earth metal extraction ability and selective separation ability which are equal to those of existing phosphorus-based compounds, and having low synthesis costs.
The solutions in Patent Documents 1 to 3 are all carried out by solvent extraction. Incidentally, the recovery of metals using ion exchange resins is widely used as well as that using solvent extraction. Unlike solvent extraction, solid extraction agents such as ion exchange resins and chelate resins have advantages of being able to be handled in a physically stable state and operated by a simple device. The reaction by ion exchange, unlike the above solvent extraction method, has less physical motion, for example, mixing an extraction agent and a solution. Contact between an ion exchange resin and a solution is generally carried out in a sealed column in many cases, and thus the capture of e.g. air is prevented. Consequently, the reaction is also characterized in that crud formation is suppressed and operations are stably carried out.
In Patent Document 4, a method for recovering scandium using a chelate resin is shown as an example of scandium recovery using a solid extraction agent. This method is a method for producing high purity scandium oxide, comprising the leaching step of obtaining a scandium-containing solution from an oxide containing minute amounts of scandium, the liquid adjusting step, the extraction step of forming a chelate resin which has adsorbed scandium, the washing step of washing the scandium-adsorbing chelate resin with a dilute acid, the back extraction step of obtaining a scandium-containing solution by eluting the scandium-adsorbing chelate resin with a strong acid, the precipitation step of obtaining scandium precipitates from the scandium-containing solution using a precipitant, and the step of calcining the precipitates.
In Patent Document 5, it is also shown that scandium is recovered using a resin impregnated with alkyl phosphonic acid or alkyl phosphoric acid ester, an extraction agent.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2000-313928
Patent Document 2: Japanese Unexamined Patent Application, Publication No. H09-291320
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2007-327085
Patent Document 4: Japanese Unexamined Patent Application, Publication No. H09-176756
Patent Document 5: Japanese Unexamined Patent Application, Publication No. H01-246328